The use of fluorescent liquid inks is well-known for the application of information to various types of documents which are intended to be processed or read by means of detection machines. Such inks are widely used for the application or cancellation of postage amounts on mail and for the indication of route codes which enable pieces of mail to be sorted and routed automatically by processing machines. Such inks are also used, alone or in combination with ordinary pigments such as carbon black, to provide images which are clearly visible to the eye and are also machine-readable to provide a double-check system which reduces the ease of fraudulently-altering checks, stock certificates, bonds and other negotiable instruments.
The conventional fluorescent inks are liquid inks which are applied to the intended documents by means of fabric printing ribbons, ink pads or postage meter pads. In many cases, different images are applied to opposite sides of the same document to provide different information on each of said sides. The ink contains an oily vehicle and a fluorescing dye which is soluble in such oil or which is present as a solid solution in a finely-particulate resinous binder material which is dispersed in the oily vehicle in the same manner as a dispersed pigment such as carbon black.
Different fluorescing dyes have different colors and emit different wave-length radiation when exposed to and excited by ultra-violet radiation. The processing machines must be adapted to recognize wave-length emissions over a relatively broad wavelength range and, therefore, must be very sensitive to the detection of even small amounts of fluorescing dyes which emit radiation over any part of the detectible wave-length range. This requirement is also necessitated by the fact that some detectible images are relatively poor in quality or are absorbed and broadened when applied to porous papers or are masked to some extent by the presence of non-fluorescing pigments such as carbon black. If the images cannot be read by the processing machine, the document is rejected and must be processed manually. In some cases, the imperfect images will be misread by the processing machine, causing errors. These defects frustrate the entire purpose of the system.
One of the most common causes of defective results in the system arises from the fact that the fluorescent inks are liquid inks which must be absorbed by the document in order to remain thereon in the form of an image which resists smudging and smearing during contact with the hands or with the processing equipment. While the images are applied as sharp, clear images, such sharpness and clarity is reduced to some extent by the absorption of the liquid ink into the document paper which causes the liquid ink to diffuse and causes the outline of the images to become uneven and fuzzy. Moreover, when different liquid ink images are applied to opposite sides of the same document, the images tend to penetrate sufficiently that they are detected by the processing equipment as objectionable or defective images when read through the opposite side, causing the document to be rejected or misread.